Comic Strips

Classic nature versus nurture debate. Assuming identical male twins, anyway. Anything else and the answer is far more likely to be "no". Bob probably has a bigger D than his twin sister Sally. Probably.

But once you narrow it down, well, they really ought to be the same unless something significant happened to one and not the other. Note that the significance of that thing might not at first appear to be significant, however.

e.g. Maybe one time and only one time, Mom or Dad fed one of them twice by accident when the twins were infants and then had to the feed the hungry one "again" when he wouldn't stop crying (if they realised the error or not).

That one extra bottle of milk with no other differences might have made for a lifetime of difference between the two. And you still can't be sure which one would end up better endowed. Maybe something latent in the twins' genes would gives a hungry baby the advantage later in life, so to speak.

ETA: If this sort of thing is your cup of tea, you might find pictures out there on the internet. If it isn't, you might want to perform the same experiment using women's attributes instead. I understand certain forms of twin "art" are quite popular.